The present invention relates generally to the field of feed-water level control in boilers, and more specifically to an automatic boiler level controller having an improved water level sensing methodology and a positive blowdown circuit to ensure that the alarm probe is free of debris or buildup.
Steam boilers are used in various applications, e.g., generation of electricity with steam turbine generators. In typical boilers a burner burns fuel from a fuel supply to create heat to generate steam from feed-water inside the boiler and the steam is piped to a generator or other system. Thus, typical boilers have both steam and feed-water inside them. It is known that the level of feed-water inside a boiler must be kept above a certain level. It is also known in the art to use a controller and one or more conductivity sensors to automatically control the level of feed-water in the boiler by controlling a boiler feed pump that provides additional feed-water to the boiler when the feed-water level falls to below a first level. It is also known to provide an alarm and/or turn off the fuel supply to the burner when the feed-water level falls to below a second, lower level.
It is known in the art of boiler level control to sense the water level inside the boiler using conductivity sensors located inside a column positioned outside the boiler but in fluid connection with the boiler. The lowest conductivity sensor is typically the alarm sensor. One known problem with this known configuration is that debris can build up around the alarm sensor causing false readings, e.g., the alarm conductivity sensor is in steam, but debris surrounding the alarm sensor provides a conductive path causing the controller to falsely determine that the alarm sensor is in water. A known solution to the debris problem is to use pressure from steam to xe2x80x9cblow downxe2x80x9d the debris away from the alarm sensor by opening a blow down valve. This blowdown procedure has the additional effect of lowering the water level in the column containing the conductivity probes, which can be problematic, because as the water is blown down from the various conductivity probes, the controller falsely determines that the level of water in the boiler is lowering and turns on the feed-water pump and/or triggers an alarm and/or shuts off the fuel to the burner. A typical way to overcome this problem is to add circuitry that bypasses the control signals from the controller during blowdown. This prevents false alarms during blowdown, but does not provide any indication as to whether the blowdown process is complete or effective.
Additionally, the circuits typically used to determine water level with conductivity probes are relatively complex with a relatively high parts count.
There is a need, therefore, for an improved boiler controller.
The present invention is directed toward an improved boiler controller. According to one aspect of the present invention, a positive blowdown signal is presented to indicate to the operator that the alarm probe is in steam; performing blowdown until this indicator turns on helps ensure that the alarm probe is working properly. According to another aspect of the present invention, the controller uses a novel methodology of sensing water level using conductivity probes. An embodiment of this methodology uses the charging and discharging of a capacitor through a resistive value to sense the presence of water.